The underlying assumption of this proposal is that T cell receptors are much like immunoglobulin heavy chains in the structural composition; T and B cells probably share V region gene products, but the constant segments of T cell receptors are probably antigenically distinct from the known immunoglobulin class markers. That T cell receptors have V regions identical with B cells has been established using anti-immunoglobulin idiotype serum to block T cell functional assays. The receptors extracted from T cells which have binding specificity for antigen are too large (100,000 MW) to be dimers of V regions alone and probably represent a V region attached to another gene product, i.e., a T cell constant region gene product. An attempt will be made to make antisera directed against constant markers on T cell receptors for defined antigens. Specifically, congenic mice will be used to look for allotypic markers on C region determinants. The objective is to (a) investigate whether there are V and C region gene products of the same molecule and (b) to compare the inheritance patterns of T and B cell allotypes. An antiserum to an allotypic determinant will be used to precipitate radiolabeled complexes of cells for peptide mapping studies to see if the molecule has chemically heterogeneous and homogenous segments. In addition, major emphasis will be given to use of any antisera developed to study the differential pathway of a T cell. It is possible that a T cell may express V region gene products linked to different C gene products at the cell surface in different stages of development.